Particle physicists might have lastly poked a gap of their understanding of the subatomic realm—which they might relish. A brand new have a look at previous information suggests an ephemeral particle known as the W boson is heavier than predicted by physicists’ “standard model” of particles and forces. The discrepancy might trace at particles not included within the 40-year-old principle, says Doreen Wackeroth, a theorist on the University at Buffalo who was not concerned within the work. “I’m very excited about the result!”
But the discovering, reported right now in Science, additionally clashes with earlier measurements, giving some physicists pause. “All these measurements claim to measure the same quantity,” says Martin Grünewald, an experimental physicist at University College Dublin. “Somebody must be, I will not say wrong, but maybe made a mistake or pushed the error evaluation too aggressively.”
Vexingly profitable, the usual mannequin was accomplished in 2012, when the world’s largest atom smasher, the Large Hadron Collider (LHC) on the European particle physics laboratory CERN, found its final lacking piece, the long-predicted Higgs boson. The principle accounts for each particle interplay seen to this point, nevertheless it suffers apparent deficiencies. It contains three forces—electromagnetic, robust, and weak— however leaves out gravity. It additionally incorporates no darkish matter, the invisible stuff that makes up 85% of the universe’s matter.
Now that each one the usual mannequin particles are recognized, physicists can take a look at the speculation’s inner consistency, as a result of every particle’s properties depend upon these of others. For instance, the mass of the W boson—which conveys the weak nuclear power simply because the photon conveys the electromagnetic power—will depend on these of the Higgs and a heavy however fleeting subatomic particle known as the highest quark. So, from these enter measurements, physicists can predict the W’s mass and search for a discrepancy with the measured worth.
The measurement is difficult. Created in a excessive vitality particle collision, a W rapidly decays into both an electron or its heavier cousin, a particle known as a muon, and an antineutrino. The antineutrino can’t be detected, so physicists should deduce its presence by summing up the momenta and energies of all the opposite particles spewing from every collision and on the lookout for occasions wherein one thing unseen appears to fly out the aspect of the cylindrical detector. From the vitality and momentum of the decay particles, analyzed statistically over many occasions, they’ll estimate the W’s mass.
Now, one group says its studying conflicts with the usual mannequin prediction. The information come from the Collider Detector at Fermi National Accelerator Laboratory (CDF), a particle detector fed by the Tevatron collider, which ran at Fermilab from 1984 till 2011. After a decade of labor, Ashutosh Kotwal, a particle physicist at Duke University, and his 397 CDF collaborators discover the W boson has a mass of 80,443.5 megaelectron volts—86 occasions that of a proton. The measurement differs from the expected mass by seven occasions the experimental uncertainty.
“What does it mean? That’s the next big question,” Wackeroth says. Physicists have noticed a few different small anomalies that counsel the usual mannequin might lastly be cracking, she says. For instance, she notes that the muon seems to be barely extra magnetic than predicted.
Weighty situation
A brand new measurement of the mass of a particle known as the W boson disagrees strongly with the theoretical prediction—and with earlier measurements, together with one from the identical group.
GRAPHIC: C. BICKEL/SCIENCE; DATA: P.A. ZYLA ET AL., PROG. THEOR. EXP. PHYS. 2020, 083C01 (2020); T. AALTONEN ET AL., PHY. REV. LETT. 108, 151803 (2012); M. AABOUD ET AL., EUR. PHYS. J. C78, 100 (2018); T. AALTONEN ET AL., SCIENCE, 376, 6589 (2022)
However, earlier measurements of the W’s mass typically agreed with the usual mannequin (see chart, under). The new outcome even contradicts the CDF’s earlier outcome, printed in 2012, which was primarily based on the primary quarter of the present information set, notes Dmitri Denisov, a physicist at Brookhaven National Laboratory who labored on D0, a rival Tevatron detector. “That’s my first concern,” he says.
But CDF researchers made a number of enhancements within the evaluation that account for the distinction, Kotwal says. “We are confident in the techniques we have used,” he says. “It is a distinct possibility that there is something new in nature that the standard model does not capture.”
Physicists ought to quickly get one more W boson mass measurement. Scientists with the Compact Muon Solenoid, a detector on the LHC, hope to publish one early subsequent 12 months, says Guillelmo Gomez-Ceballos, a CMS physicist on the Massachusetts Institute of Technology. He can also be a CDF member, and though he didn’t work on the brand new research, he says, “I don’t remember any analysis that has been done with so much care.”
It might take years to reconcile the measurements. But physicists gained’t be left rudderless within the meantime. Since 1957, the Particle Data Group (PDG) at Lawrence Berkeley National Laboratory (LBNL) has maintained a compendium of particles and arbitrated disputes over their measured properties. The new W boson mass worth comes because the PDG is getting ready its newest annual replace, says Michael Barnett, a retired LBNL physicist who led the PDG from 1990 to 2015 and nonetheless works on it. “We’re going to have to stop the presses, just like we did when the Higgs was discovered,” he says.
For a parameter just like the W boson’s mass, the PDG averages essentially the most present and dependable measurements. If they disagree far past their uncertainties, the group applies a selected mathematical algorithm that successfully widens the error bars to embody the discordant particular person outcomes, Barnett says. Ironically, although the CDF has now reported the only most exact measurement of the W mass, the official worth will probably turn into even much less sure than earlier than.